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<a name="typedef-arelent"></a>
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<p>
Next: <a href="howto-manager.html#howto-manager" accesskey="n" rel="next">howto manager</a>, Previous: <a href="Relocations.html#Relocations" accesskey="p" rel="prev">Relocations</a>, Up: <a href="Relocations.html#Relocations" accesskey="u" rel="up">Relocations</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="BFD-Index.html#BFD-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="typedef-arelent-1"></a>
<h4 class="subsection">2.10.1 typedef arelent</h4>
<p>This is the structure of a relocation entry:
</p>

<div class="example">
<pre class="example">

typedef enum bfd_reloc_status
{
  /* No errors detected.  Note - the value 2 is used so that it
     will not be mistaken for the boolean TRUE or FALSE values.  */
  bfd_reloc_ok = 2,

  /* The relocation was performed, but there was an overflow.  */
  bfd_reloc_overflow,

  /* The address to relocate was not within the section supplied.  */
  bfd_reloc_outofrange,

  /* Used by special functions.  */
  bfd_reloc_continue,

  /* Unsupported relocation size requested.  */
  bfd_reloc_notsupported,

  /* Unused.  */
  bfd_reloc_other,

  /* The symbol to relocate against was undefined.  */
  bfd_reloc_undefined,

  /* The relocation was performed, but may not be ok.  If this type is
     returned, the error_message argument to bfd_perform_relocation
     will be set.  */
  bfd_reloc_dangerous
 }
 bfd_reloc_status_type;

typedef const struct reloc_howto_struct reloc_howto_type;

typedef struct reloc_cache_entry
{
  /* A pointer into the canonical table of pointers.  */
  struct bfd_symbol **sym_ptr_ptr;

  /* offset in section.  */
  bfd_size_type address;

  /* addend for relocation value.  */
  bfd_vma addend;

  /* Pointer to how to perform the required relocation.  */
  reloc_howto_type *howto;

}
arelent;

</pre></div>
<p><strong>Description</strong><br>
Here is a description of each of the fields within an <code>arelent</code>:
</p>
<ul>
<li> <code>sym_ptr_ptr</code>
</li></ul>
<p>The symbol table pointer points to a pointer to the symbol
associated with the relocation request.  It is the pointer
into the table returned by the back end&rsquo;s
<code>canonicalize_symtab</code> action. See <a href="Symbols.html#Symbols">Symbols</a>. The symbol is
referenced through a pointer to a pointer so that tools like
the linker can fix up all the symbols of the same name by
modifying only one pointer. The relocation routine looks in
the symbol and uses the base of the section the symbol is
attached to and the value of the symbol as the initial
relocation offset. If the symbol pointer is zero, then the
section provided is looked up.
</p>
<ul>
<li> <code>address</code>
</li></ul>
<p>The <code>address</code> field gives the offset in bytes from the base of
the section data which owns the relocation record to the first
byte of relocatable information. The actual data relocated
will be relative to this point; for example, a relocation
type which modifies the bottom two bytes of a four byte word
would not touch the first byte pointed to in a big endian
world.
</p>
<ul>
<li> <code>addend</code>
</li></ul>
<p>The <code>addend</code> is a value provided by the back end to be added (!)
to the relocation offset. Its interpretation is dependent upon
the howto. For example, on the 68k the code:
</p>
<div class="example">
<pre class="example">        char foo[];
        main()
                {
                return foo[0x12345678];
                }
</pre></div>

<p>Could be compiled into:
</p>
<div class="example">
<pre class="example">        linkw fp,#-4
        moveb @#12345678,d0
        extbl d0
        unlk fp
        rts
</pre></div>

<p>This could create a reloc pointing to <code>foo</code>, but leave the
offset in the data, something like:
</p>
<div class="example">
<pre class="example">RELOCATION RECORDS FOR [.text]:
offset   type      value
00000006 32        _foo

00000000 4e56 fffc          ; linkw fp,#-4
00000004 1039 1234 5678     ; moveb @#12345678,d0
0000000a 49c0               ; extbl d0
0000000c 4e5e               ; unlk fp
0000000e 4e75               ; rts
</pre></div>

<p>Using coff and an 88k, some instructions don&rsquo;t have enough
space in them to represent the full address range, and
pointers have to be loaded in two parts. So you&rsquo;d get something like:
</p>
<div class="example">
<pre class="example">        or.u     r13,r0,hi16(_foo+0x12345678)
        ld.b     r2,r13,lo16(_foo+0x12345678)
        jmp      r1
</pre></div>

<p>This should create two relocs, both pointing to <code>_foo</code>, and with
0x12340000 in their addend field. The data would consist of:
</p>
<div class="example">
<pre class="example">RELOCATION RECORDS FOR [.text]:
offset   type      value
00000002 HVRT16    _foo+0x12340000
00000006 LVRT16    _foo+0x12340000

00000000 5da05678           ; or.u r13,r0,0x5678
00000004 1c4d5678           ; ld.b r2,r13,0x5678
00000008 f400c001           ; jmp r1
</pre></div>

<p>The relocation routine digs out the value from the data, adds
it to the addend to get the original offset, and then adds the
value of <code>_foo</code>. Note that all 32 bits have to be kept around
somewhere, to cope with carry from bit 15 to bit 16.
</p>
<p>One further example is the sparc and the a.out format. The
sparc has a similar problem to the 88k, in that some
instructions don&rsquo;t have room for an entire offset, but on the
sparc the parts are created in odd sized lumps. The designers of
the a.out format chose to not use the data within the section
for storing part of the offset; all the offset is kept within
the reloc. Anything in the data should be ignored.
</p>
<div class="example">
<pre class="example">        save %sp,-112,%sp
        sethi %hi(_foo+0x12345678),%g2
        ldsb [%g2+%lo(_foo+0x12345678)],%i0
        ret
        restore
</pre></div>

<p>Both relocs contain a pointer to <code>foo</code>, and the offsets
contain junk.
</p>
<div class="example">
<pre class="example">RELOCATION RECORDS FOR [.text]:
offset   type      value
00000004 HI22      _foo+0x12345678
00000008 LO10      _foo+0x12345678

00000000 9de3bf90     ; save %sp,-112,%sp
00000004 05000000     ; sethi %hi(_foo+0),%g2
00000008 f048a000     ; ldsb [%g2+%lo(_foo+0)],%i0
0000000c 81c7e008     ; ret
00000010 81e80000     ; restore
</pre></div>

<ul>
<li> <code>howto</code>
</li></ul>
<p>The <code>howto</code> field can be imagined as a
relocation instruction. It is a pointer to a structure which
contains information on what to do with all of the other
information in the reloc record and data section. A back end
would normally have a relocation instruction set and turn
relocations into pointers to the correct structure on input -
but it would be possible to create each howto field on demand.
</p>
<a name="enum-complain_005foverflow"></a>
<h4 class="subsubsection">2.10.1.1 <code>enum complain_overflow</code></h4>
<p>Indicates what sort of overflow checking should be done when
performing a relocation.
</p>

<div class="example">
<pre class="example">

enum complain_overflow
{
  /* Do not complain on overflow.  */
  complain_overflow_dont,

  /* Complain if the value overflows when considered as a signed
     number one bit larger than the field.  ie. A bitfield of N bits
     is allowed to represent -2**n to 2**n-1.  */
  complain_overflow_bitfield,

  /* Complain if the value overflows when considered as a signed
     number.  */
  complain_overflow_signed,

  /* Complain if the value overflows when considered as an
     unsigned number.  */
  complain_overflow_unsigned
};
</pre></div>
<a name="reloc_005fhowto_005ftype"></a>
<h4 class="subsubsection">2.10.1.2 <code>reloc_howto_type</code></h4>
<p>The <code>reloc_howto_type</code> is a structure which contains all the
information that libbfd needs to know to tie up a back end&rsquo;s data.
</p>

<div class="example">
<pre class="example">struct reloc_howto_struct
{
  /* The type field has mainly a documentary use - the back end can
     do what it wants with it, though normally the back end's idea of
     an external reloc number is stored in this field.  */
  unsigned int type;

  /* The encoded size of the item to be relocated.  This is *not* a
     power-of-two measure.  Use bfd_get_reloc_size to find the size
     of the item in bytes.  */
  unsigned int size:3;

  /* The number of bits in the field to be relocated.  This is used
     when doing overflow checking.  */
  unsigned int bitsize:7;

  /* The value the final relocation is shifted right by.  This drops
     unwanted data from the relocation.  */
  unsigned int rightshift:6;

  /* The bit position of the reloc value in the destination.
     The relocated value is left shifted by this amount.  */
  unsigned int bitpos:6;

  /* What type of overflow error should be checked for when
     relocating.  */
  ENUM_BITFIELD (complain_overflow) complain_on_overflow:2;

  /* The relocation value should be negated before applying.  */
  unsigned int negate:1;

  /* The relocation is relative to the item being relocated.  */
  unsigned int pc_relative:1;

  /* Some formats record a relocation addend in the section contents
     rather than with the relocation.  For ELF formats this is the
     distinction between USE_REL and USE_RELA (though the code checks
     for USE_REL == 1/0).  The value of this field is TRUE if the
     addend is recorded with the section contents; when performing a
     partial link (ld -r) the section contents (the data) will be
     modified.  The value of this field is FALSE if addends are
     recorded with the relocation (in arelent.addend); when performing
     a partial link the relocation will be modified.
     All relocations for all ELF USE_RELA targets should set this field
     to FALSE (values of TRUE should be looked on with suspicion).
     However, the converse is not true: not all relocations of all ELF
     USE_REL targets set this field to TRUE.  Why this is so is peculiar
     to each particular target.  For relocs that aren't used in partial
     links (e.g. GOT stuff) it doesn't matter what this is set to.  */
  unsigned int partial_inplace:1;

  /* When some formats create PC relative instructions, they leave
     the value of the pc of the place being relocated in the offset
     slot of the instruction, so that a PC relative relocation can
     be made just by adding in an ordinary offset (e.g., sun3 a.out).
     Some formats leave the displacement part of an instruction
     empty (e.g., ELF); this flag signals the fact.  */
  unsigned int pcrel_offset:1;

  /* src_mask selects the part of the instruction (or data) to be used
     in the relocation sum.  If the target relocations don't have an
     addend in the reloc, eg. ELF USE_REL, src_mask will normally equal
     dst_mask to extract the addend from the section contents.  If
     relocations do have an addend in the reloc, eg. ELF USE_RELA, this
     field should normally be zero.  Non-zero values for ELF USE_RELA
     targets should be viewed with suspicion as normally the value in
     the dst_mask part of the section contents should be ignored.  */
  bfd_vma src_mask;

  /* dst_mask selects which parts of the instruction (or data) are
     replaced with a relocated value.  */
  bfd_vma dst_mask;

  /* If this field is non null, then the supplied function is
     called rather than the normal function.  This allows really
     strange relocation methods to be accommodated.  */
  bfd_reloc_status_type (*special_function)
    (bfd *, arelent *, struct bfd_symbol *, void *, asection *,
     bfd *, char **);

  /* The textual name of the relocation type.  */
  const char *name;
};

</pre></div>
<a name="index-The-HOWTO-Macro"></a>
<a name="The-HOWTO-Macro"></a>
<h4 class="subsubsection">2.10.1.3 <code>The HOWTO Macro</code></h4>
<p><strong>Description</strong><br>
The HOWTO macro fills in a reloc_howto_type (a typedef for
const struct reloc_howto_struct).
</p><div class="example">
<pre class="example">#define HOWTO(type, right, size, bits, pcrel, left, ovf, func, name,   \
              inplace, src_mask, dst_mask, pcrel_off)                  \
  { (unsigned) type, size &lt; 0 ? -size : size, bits, right, left, ovf,  \
    size &lt; 0, pcrel, inplace, pcrel_off, src_mask, dst_mask, func, name }
</pre></div>

<p><strong>Description</strong><br>
This is used to fill in an empty howto entry in an array.
</p><div class="example">
<pre class="example">#define EMPTY_HOWTO(C) \
  HOWTO ((C), 0, 0, 0, FALSE, 0, complain_overflow_dont, NULL, \
         NULL, FALSE, 0, 0, FALSE)

</pre></div>

<a name="index-bfd_005fget_005freloc_005fsize"></a>
<a name="bfd_005fget_005freloc_005fsize"></a>
<h4 class="subsubsection">2.10.1.4 <code>bfd_get_reloc_size</code></h4>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">unsigned int bfd_get_reloc_size (reloc_howto_type *);
</pre></div>
<p><strong>Description</strong><br>
For a reloc_howto_type that operates on a fixed number of bytes,
this returns the number of bytes operated on.
</p>
<a name="index-arelent_005fchain"></a>
<a name="arelent_005fchain"></a>
<h4 class="subsubsection">2.10.1.5 <code>arelent_chain</code></h4>
<p><strong>Description</strong><br>
How relocs are tied together in an <code>asection</code>:
</p><div class="example">
<pre class="example">typedef struct relent_chain
{
  arelent relent;
  struct relent_chain *next;
}
arelent_chain;

</pre></div>

<a name="index-bfd_005fcheck_005foverflow"></a>
<a name="bfd_005fcheck_005foverflow"></a>
<h4 class="subsubsection">2.10.1.6 <code>bfd_check_overflow</code></h4>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">bfd_reloc_status_type bfd_check_overflow
   (enum complain_overflow how,
    unsigned int bitsize,
    unsigned int rightshift,
    unsigned int addrsize,
    bfd_vma relocation);
</pre></div>
<p><strong>Description</strong><br>
Perform overflow checking on <var>relocation</var> which has
<var>bitsize</var> significant bits and will be shifted right by
<var>rightshift</var> bits, on a machine with addresses containing
<var>addrsize</var> significant bits.  The result is either of
<code>bfd_reloc_ok</code> or <code>bfd_reloc_overflow</code>.
</p>
<a name="index-bfd_005freloc_005foffset_005fin_005frange"></a>
<a name="bfd_005freloc_005foffset_005fin_005frange"></a>
<h4 class="subsubsection">2.10.1.7 <code>bfd_reloc_offset_in_range</code></h4>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">bfd_boolean bfd_reloc_offset_in_range
   (reloc_howto_type *howto,
    bfd *abfd,
    asection *section,
    bfd_size_type offset);
</pre></div>
<p><strong>Description</strong><br>
Returns TRUE if the reloc described by <var>HOWTO</var> can be
applied at <var>OFFSET</var> octets in <var>SECTION</var>.
</p>
<a name="index-bfd_005fperform_005frelocation"></a>
<a name="bfd_005fperform_005frelocation"></a>
<h4 class="subsubsection">2.10.1.8 <code>bfd_perform_relocation</code></h4>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">bfd_reloc_status_type bfd_perform_relocation
   (bfd *abfd,
    arelent *reloc_entry,
    void *data,
    asection *input_section,
    bfd *output_bfd,
    char **error_message);
</pre></div>
<p><strong>Description</strong><br>
If <var>output_bfd</var> is supplied to this function, the
generated image will be relocatable; the relocations are
copied to the output file after they have been changed to
reflect the new state of the world. There are two ways of
reflecting the results of partial linkage in an output file:
by modifying the output data in place, and by modifying the
relocation record.  Some native formats (e.g., basic a.out and
basic coff) have no way of specifying an addend in the
relocation type, so the addend has to go in the output data.
This is no big deal since in these formats the output data
slot will always be big enough for the addend. Complex reloc
types with addends were invented to solve just this problem.
The <var>error_message</var> argument is set to an error message if
this return <code>bfd_reloc_dangerous</code>.
</p>
<a name="index-bfd_005finstall_005frelocation"></a>
<a name="bfd_005finstall_005frelocation"></a>
<h4 class="subsubsection">2.10.1.9 <code>bfd_install_relocation</code></h4>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">bfd_reloc_status_type bfd_install_relocation
   (bfd *abfd,
    arelent *reloc_entry,
    void *data, bfd_vma data_start,
    asection *input_section,
    char **error_message);
</pre></div>
<p><strong>Description</strong><br>
This looks remarkably like <code>bfd_perform_relocation</code>, except it
does not expect that the section contents have been filled in.
I.e., it&rsquo;s suitable for use when creating, rather than applying
a relocation.
</p>
<p>For now, this function should be considered reserved for the
assembler.
</p>

<hr>
<div class="header">
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